1. Field of the Invention
The present invention relates to a permanent magnet (PM) type stepping motor and, more particularly, configuration of magnetic poles of a rotor.
2. Description of the Prior Art
In recent years, as the performances of various devices such as a printer, a facsimile, and a floppy disk drive become enhanced, a stepping motor is increasingly required to have better performance such as higher torque and better quality such as lower vibration realized by reducing cogging. Although a PM type stepping motor used in various devices has been improved in order to meet such requirements, it is difficult to satisfy both high torque and low vibration which are mutually contradictory.
In a PM stepping motor, a rotor unit is rotatably disposed inside a stator unit and is rotated by interaction between respective magnetic fluxes of the stator unit and the rotor unit. FIG. 1A is a perspective view of a conventional rotor unit 29, and FIG. 1B shows a developed pattern of S poles and N poles magnetized on the circumference of the rotor unit 29.
As shown in FIG. 1A, in the rotor unit 29, a holder 27 is press fitted onto a shaft 23 as a central axis of rotation, and a magnet 24 whose S and N poles are arranged at predetermined intervals in the circumferential direction thereof thereby forming the developed pattern of magnetic poles shown in FIG. 1B is fixed onto the circumferential face of the holder 27 with the holder 27.
As shown in FIG. 1B, the S pole and the N pole are alternately magnetized in such a manner as to have the same width (W1=W2), whereby cogging occurs when a magnetic flux fluctuates.
In a general lamination type motor in which phases are arranged in the circumferential direction, the cogging can be reduced by skewing the magnetic poles of the rotor, or some other methods as well. However, in the PM stepping motor in which phases are arranged in the axial direction, there is almost no countermeasure except skewing the shape of pole teeth, making it difficult to reduce the cogging.
In skewing the shape of the pole teeth for reducing the cogging, the skewing work is carried out by processing a sheet metal, so a die is necessary for prototyping, which requires a lot of expense and time and, moreover, which makes it difficult to maintain precision. Therefore, the cogging could not be easily reduced.
The invention has been made in consideration of the above and it is therefore an object of the invention to provide a stepping motor of low vibration realized by reducing cogging with a simple structure while maintaining the performances such as torque requirements.
According to the invention, in a stepping motor in which a stator unit is composed of a pair of stator sub-assemblies attached to each other in a back to back manner, each stator sub-assembly having a plurality of pole teeth formed at its inner circumference and housing a coil inside thereof, and a rotor unit is rotatably disposed with a small gap from the plurality of pole teeth and has multiple magnetic poles formed on a circumference thereof, the multiple magnetic poles of the rotor unit being formed by magnetizing the rotor unit alternately with an S pole and an N pole in the circumferential direction, a magnetic pole width consisting of a width of the S pole and a width of the N pole in a pair is set to a predetermined constant value with the width of the S pole and the width of the N pole in each pair set to be different from each other.
The width of the S pole and the width of the N pole in each pair are different from each other by an electrical angle ranging from 15 degrees to 50 degrees.
Further, one pair of the S pole and the N pole in which the width of the S pole is set to be smaller than the width of the N pole and another pair of the S pole and the N pole in which the width of the S pole is set to be larger than the width of the N pole are alternately arranged.